The mean sea level (in a geocentric coordinate system) can beadequately measured by satellite altimetry. In order tomake the product useful for oceanography the altimetricsurface must be referenced to an equipotential surface,the geoid, to obtain the 'dynamical height'.This heigt can be blended with that of an ocean generalcirculation model to derive an optimal dynamic sea surface.Likewise optimal values for altimetry and the geoid can be found.The latter can be validated against results ofnew gravity missions CHAMP and GRACE.To understand local trends a separate but closely relatedstudy is performed. Hydrographic measurements and sea surfaceheight anomalies over the last decade are assimilated into anocean general circulation model using the 4DVAR technique.The ocean model conserves mass (salt) rather than volume.Steric expansion due to changes in temperature or salinityare taken into account.Results are presented for global and local sea level trendsaccording to mass changes, volume changes and redistributionby changes of the circulation pattern.Additionally a related hindcast experiment for 1950 to 1999will be analyzed in the same manner.